1. Field of the Invention
The present invention relates to a device for interconnection and protection of a microwave integrated circuit chip formed of III-IV materials such as GaAs. The performances of a microwave integrated circuit mounted in said device are not reduced by this latter over a range of frequencies extending from the d.c. level to at least 40 Ghz and up to 60 GHz if the substrate is matched.
2. Description of the Prior Art
It is known that microwave integrated circuits, often known as MMICs for Microwave Monolithic Integrated Circuits, are difficult to use.
In the first place, these integrated circuits are very generally of GaAs or of comparable materials such as AlGaAs, InP, . . . , and have a surface Si.sub.3 N.sub.4 which does not provide optimum protection as in the case of a silicon circuit. They accordingly have to be protected against climato-mechanical stresses by an encapsulating package.
In addition, however, it is also known that a MMIC which operates well in the state of a bare chip suffers from considerable degradation of its characteristics when it is encapsulated within a package. Many parameters are involved, including the volume of the cavity of the package and the passage of the access connections through the package walls. These latter are either coaxial connections formed by glass beads (which is illogical since the circuit associated with a MMIC is very generally of microstrips) or flat connections which pass through the package in the plane of bonding of the cap on the base within the thickness of the glass layer but which exhibit microwave leakages.
Results which are nevertheless acceptable are obtained with complex, bulky and costly packages but:
these latter are not compatible with a microstrip circuit environment, PA1 they are not cascadable, this type of assembly being frequently employed in microwave circuits, PA1 they have relatively high insertion loss by reason of their system of access connections. PA1 a thin substrate of dielectric material, a first face of which supports a network of microstrip lines and a second face of which supports a ground plane metallization for the microstrip lines of the first face, PA1 at least one chip of microwave semiconductor component fixed on the ground plane metallization of the second face of the substrate, which is connected electrically to the microstrip lines of the first face of the substrate by means of plugged metallized holes, PA1 a metallic sole-piece in which a recess provides a housing for the semiconductor component fixed on the second face of the substrate, the ground plane metallization of which is brazed on the sole-piece by means of a brazing preform.
The device in accordance with the invention provides a solution to these problems of environment of a MMIC chip by adopting as a starting element a thin ceramic substrate which alone constitutes a portion of a microstrip circuit. In other words, this ceramic substrate supports the MMIC chip and the microstrip lines which serve to interconnect this element with other comparable elements (mounted in cascade) or with an external circuit of microstrips.
However, since the MMIC chip cannot be protected by a glass layer or a polymer drop, the chip is no longer placed, as in the prior art, on that face of the substrate which carries the microstrip interconnection and which normally constitutes the top face of a microwave circuit. On the contrary, the chip is placed beneath the substrate and protected by the fastening sole-piece of the device which also serves as a radiator, in which a local reduction in thickness offers a compartment for housing the MMIC chip and any other requisite components such as capacitors.
The MMIC chip and the other components which may be provided are connected to the network of microstrips of the top face by means of metallized holes which are hermetically plugged.
Thus the device in accordance with the invention is essentially constituted by a substrate etched on a double face, which carries on its bottom face at least one microwave component in the form of a bare chip protected by the fastening sole-piece in which is formed a compartment, and on its top face at least one network of microstrips and if necessary components such as capacitors, inductors and chips of glassivated silicon, the two faces being interconnected by means of metallized holes plugged with brazing alloy. I is readily apparent that the sole-piece is hermetically brazed on the double-face substrate.
The invention which has been primarily developed for integrated circuits applies also to microwave transistors.